1. Field of the Invention
The present invention relates to an adhesive composition, an adhesive optical film, and an image display device. More particularly, it relates to an adhesive composition, an adhesive optical film used as various optical films comprising an adhesive layer and an under coat layer, and an image display device.
2. Description of the Background Art
Heretofore, optical films such as polarizing film, phase difference film, luminance improving film, and view-angle expansion film have been used for various industrial purposes and, for example, these optical films are used for sticking onto image display devices such as liquid crystal display, organic electroluminescence device (organic EL display device), and plasma display panel (PDP).
As the optical film to be sticked onto a liquid crystal display, an adhesive optical film comprising an optical film laminated with an adhesive is known.
For example, there is proposed an adhesive optical film in which an isocyanate-based polyfunctional compound is added to an acrylic polymer solution to prepare an adhesive solution and layer made of the resulting adhesive solution is laminated on a polarizing film comprising an under coat layer (anchor layer) made of a water dispersible polyester-based resin (see, for example, Japanese Unexamined Patent Publication No. 2004-54007).
For example, there is also proposed an adhesive optical film in which an adhesive layer is laminated on an optical film with an under coat layer made of a polyamine compound interposed (see, for example, Japanese Unexamined Patent Publication No. 2004-78143).
Further, for example, there is also proposed an adhesive polarizing plate in which an acrylic adhesive layer is laminated on a polarizing plate with an under coat layer made of a polyacrylic ester having a primary amino group (see, for example, Japanese Unexamined Patent Publication No. 10-20118).
In view of the environmental burden, it has recently been required to reduce the use of an organic solvent, and a solvent-based adhesive containing an organic solvent as a solvent is replaced by a water dispersible adhesive containing water as a dispersion medium.
As such the water dispersible adhesive, for example, there is proposed a pressure sensitive adhesive composition comprising a copolymer emulsion in which a copolymer contains 2-ethylhexyl methacrylate of the amount of 10 to 50% by weight based on the entire copolymer and the copolymer has a glass transition temperature of −25° C. or lower (see, for example, Japanese Unexamined Patent Publication No. 2001-254063).
However, conventional adhesives including those disclosed in the above-mentioned Japanese Unexamined Patent Publication No. 2004-54007, Japanese Unexamined Patent Publication No. 2004-78143 and Japanese Unexamined Patent Publication No. 10-20118 show low adhesion with an optical film. When an optical film is sticked onto a liquid crystal display, the optical film is usually sticked onto the liquid crystal display and, if necessary, the optical film is once removed for positioning and then sticked again (reworking). However, these adhesives are remained on the surface of the liquid crystal display when removed (hereinafter referred to as “adhesive residue”), and thus reworkability is insufficient.
When the terminal portion of an adhesive optical film contacts with persons or articles in the vicinity in case of handling in the step of cutting or conveying the adhesive optical film, the adhesive is removed at the contact portion (hereinafter referred to as “lack of adhesive”). When the adhesive optical film, which caused lack of adhesive, is attached onto a liquid crystal display, a problem such as poor display occurs.
Conventional water dispersible adhesives including those disclosed in the above-mentioned Japanese Unexamined Patent Publication No. 2001-254063 show particularly low adhesion to a hydrophilic adherend such as glass and therefore it has difficulty to firmly stick to a glass substrate of a liquid crystal display, although adhesion to a hydrophobic adherend such as polyolefin is improved.
Furthermore, excellent heat resistance, which does not cause defects due to severe heating, is required for an adhesive to be laminated for an adhesive optical film. According to the purposes, the adhesive may be used under not only severe heating conditions but also severe heating and humidifying conditions, and thus excellent moist heat resistance is also required.
With respect to a water dispersible adhesive to be laminated on an optical film, heat resistance and moisture resistance, which does not cause deterioration of adhesion due to severe heating and humidifying, are required.